1. Field of the Invention
The present invention relates to an apparatus for detecting the abnormal state of a machine tool so as to protect the machine tool from an impact caused by an abnormality in the operation and, more particularly, to an abnormal state detecting apparatus of a machine tool having a feedback circuit.
2. Description of the Related Art
Numerically controlled machine tools such as a numerically controlled lathe which are operated for a predetermined machining while being numerically controlled in accordance with NC program data are widely used.
Such a numerically controlled machine tool, however, has some problems. For example, when a work hits against the tool or another part due to an error in a machining program or an error in the operation, serious influence is exerted on the machining accuracy, or the work, the tool or the machine is sometimes damaged.
For example, such a problem was actually brought about when the tool or the tool rest hit against the spindle while it was being fed to the feed shaft mechanism.
To avoid such a problem, the feed shaft mechanism with a mechanical overload preventing device such as a shear pin or a torque limiter is conventionally provided so as to lighten the impact applied to the important portion such as the work and the machine by collision. Some feed shaft mechanisms are provided with an emergency stopping device for constantly monitoring the acceleration of the feed shaft mechanism so as to detect an abnormal acceleration due to a collision and stop the machine in case of emergency.
FIG. 5 is a schematic side elevational view of the structure of the feed shaft mechanism of a lathe as an example of such a numerically controlled machine tool.
In FIG. 5, the lathe is composed of a spindle 10, a work 12 held by the chuck of the spindle 10, a cutting tool 14 for cutting the work 12, a tool rest 16 for fixing the cutting tool 14, a saddle 18 (traverse table) for positioning the cutting tool 14 by feeding it in the transverse direction to the position for cutting the work 12, a servo-motor 20 as a driving source for moving the saddle 18 itself at predetermined speed and thrust (torque) and a ball screw 22 which is connected to the rotary shaft 20a of the servo-motor 20.
A coupling 24 is provided between the ball screw 22 and the rotary shaft 20a of the servomotor 20. The coupling 24 is a collision detecting device such as a mechanical overload preventing device or an emergency stopping device.
In a lathe having the above-described structure, when the cutting tool 14 or the tool rest 16 hits against the work 12 due to an error in operation, an error in programming or the like, the impact of collision is transmitted from the tool rest 16 to the servomotor 20 through the saddle 18, the ball screw 22 and the coupling 24.
Therefore, the impact between the coupling 24 and the servomotor 20 is detected by the collision detecting device provided therebetween so as to prevent the torque from being transmitted and to protect the work 12 and the cutting tool 14 from the impact.
FIG. 6 shows the coupling 24 which is the mechanical overload preventing device provided as a collision detecting device in FIG. 5. The principle of the operation thereof will be explained hereinunder.
In FIG. 6, the mechanical overload preventing device is composed of a shear pin 26. When a collision is caused, namely, the cutting tool 14, the tool rest 16, the saddle 18 or the like at the end of the feed shaft system hits against the work 12 and an overload is applied to the ball screw 22 through the saddle 18, the ball screw 22 is suddenly stopped.
However, since the rotary shaft 20a of the servomotor 20 tends to continue the rotation in opposition to the stopping operation of the ball screw 22, the shear pin 26 itself is sheared by the thrust of the rotary shaft 20a, whereby the torque is not transmitted between the servomotor 20 and the ball screw 22.
In this way, the mechanical overload preventing device suppresses the damage of the feed shaft system and the influence of a collision on the mechanical accuracy to the minimum by preventing the torque from being transmitted from the servomotor 20 to the ball screw 22.
The operation of a conventional emergency stopping device which detects a collision from the acceleration will now be explained with reference to FIGS. 7 and 8.
FIG. 7 shows the time constant in the states of accelerating and decelerating the feed shaft system of a machine tool during the normal operation, and FIG. 8 shows the time constant during the collision of the feed shaft system.
This type of emergency stopping device for detecting a collision from a change in the speed is so controlled that the time constant at the time of acceleration and deceleration of the feed shaft mechanism during the normal operation is constant.
As shown in FIG. 8, the speeds Vs1 and Vs2 at a suspension during the normal operation are lowered at a predetermined rate and the change in the speed is constant.
However, when the tool 14 or the tool rest 16 hits against the work 12 or another part, the speed or a change in the speed (acceleration) is different from that in the normal state, as shown in FIG. 8. In other words, the deceleration rate shows an abnormality.
In FIG. 8, the speeds Vc1, Vc2 at the time of a collision are distinctly different from the speeds Vs1, Vs2 in the normal state. That is, the change in the speed is not constant at the time of a collision.
Therefore, in a conventional emergency stopping device for stopping the machine when a collision is detected, the breakage due to a collision of a machine system is suppressed to the minimum by stopping the feed shaft system merely on the basis of the detected change in the speed (acceleration) at the time of a collision.
However, a collision detecting device having such a conventional mechanical overload preventing device in a numerically controlled machine tool has the following designing and manufacturing problems.
(1) The load for shearing the shear pin 26 is unstable due to the nonuniformity of the quality of the shear pin 26, namely, the nonuniformity of the machining accuracy of the shear pin 26.
(2) Since a load is constantly applied to the shear pin 26 when the feed shaft is moved, the shear pin 26 is sometimes sheared even during the normal operation due to a change with time and fatigue after a long-term use, thereby actuating the mechanical overload preventing device itself.
(3) Since the shear pin 26 is sheared by a mechanical system, it is difficult to set the accurate value of the load which actuates the mechanical overload preventing device.
(4) It is necessary to install the collision detecting device in the interior of a machine tool due to the structure of the machine tool itself. It is therefore necessary to take out the device for replacing the shear pin 26 which is sheared by the operation of the device at the time of a collision and restore it to the original place after replacement. This operation takes much time and labor.
(5) The mechanical system of the device increases the number of necessary parts and assembly steps and, hence, raises the manufacturing cost.
(6) The step for aligning the ball screw 22 and the rotary shaft 20a coaxially with each other when disposing the coupling 24 must be performed anew.
On the other hand, in a conventional emergency stopping device for detecting a collision only from the acceleration, especially, in the case of cutting, the speed (acceleration) of the feed shaft mechanism is comparatively constant with a small change in a light cutting or a continuous cutting of a round material in which the amount of cutting and the amount of feeding are small.
However, in a heavy cutting or an intermittent cutting of a square material in which the cutting conditions rapidly change, since the speed of the feed shaft also rapidly changes (a change in the speed is not constant), discrimination between the normal cutting and an abnormal cutting (collision) is very difficult.
Therefore, the accurate detection of a collision is impossible in such a conventional emergency stopping device for detecting a collision only from the acceleration.
That is, the emergency stopping device is lacking in reliability.